1. Field of the Invention
The invention relates to a valve characteristic changing apparatus for an internal combustion engine that is provided with a plurality of varying mechanisms that separately vary a valve characteristic of a plurality of engine valves.
2. Description of the Related Art
A variable valve timing mechanism is widely used in internal combustion engines for vehicles and the like for the purpose of improving engine performance, including output performance, fuel consumption performance, and exhaust performance. A variable valve timing mechanism varies the opening timing and/or closing timing of an engine valve, a so-called valve timing, in accordance with the engine operating condition. Many such variable valve timing mechanisms are drivingly connected to a crankshaft serving as an engine output shaft, and driven by hydraulic pressure generated from a hydraulic pump that operates in accordance with the rotation of the crankshaft.
In recent years, valve timing changing apparatuses have been proposed which provide a plurality of variable valve timing mechanisms. For example, there is an apparatus that separately provides a variable valve timing mechanism such as described above for an intake valves and exhaust valves (refer to Japanese Patent Laid-Open Publication No. 2000-110527 (JP-A-2000-110527) for an example), and an apparatus that separately provides the variable valve timing mechanism for intake valves and exhaust valves in each bank of a V-shaped internal combustion engine.
Normally in the above valve timing changing apparatus, the hydraulic pressure generated from the hydraulic pump is distributed and supplied to the plurality of variable valve timing mechanisms. Each variable valve timing mechanism then operates based upon the supplied hydraulic pressure. In addition, the operation of each variable valve timing mechanism in the valve timing change apparatus is often synchronized in order to change the valve timing in accordance with the engine operating condition.
Therefore, it is difficult in the above apparatus to supply sufficient hydraulic pressure to each variable valve timing mechanism during times of change in the engine operating condition. Moreover, this is likely to decrease operation response. In cases where a hydraulic pump with high output performance is used to improve operation response, such a pump increases size and is thus not preferable.
It should be noted that the aforementioned problems occur not only in an apparatus provided with a plurality of hydraulic pump-driven variable valve timing mechanisms as described in detail above, but also in other valve timing changing apparatus provided with a plurality of variable valve timing mechanisms that are operated via the same drive train, such as a plurality of electrically-driven variable valve timing mechanisms. Furthermore, the above description pertains to a variable valve timing mechanism that changes a valve timing; however, related art also includes that with substantially the same problem in a varying mechanism that changes a so-called valve characteristic, for example, a lift amount or the like.